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IO-360 (180hp) #1 cylinder at 5/80

Carl Froehlich

Well Known Member
I'm helping a neighbor work and engine problem. The plane has been idle for several months and he is getting it ready for conditional inspection. The compression check for #1 cylinder is 5/80, the other three cylinders are in the mid 70s. There is perhaps 300 hours on this engine.

Bore scope showed nothing of interest. The valves and valve seats look fine, and both valves are fully closing. The cylinder walls looked normal. I could find no cracks on the cylinder head or barrel (some areas are obscured).

On engine run up the engine is smooth and all four cylinders CHT and EGTs are tracking (as in normal temps). After a long high (for the ground) RPM run we did the compression check again and it was still at 5/80.

I can hear the air leaking out of the cylinder and noted that a little air was coming out the #3 cylinder spark plug hole (plugs were removed for compression checks).

My only guess are the rings have failed - but I would have expected to see some evidence of this on the cylinder walls.

Other ideas?
Recommendations for next step?

Carl
 
Possibly the ring gaps have aligned? The combustion cycle is fast enough to cover that up, but a compression check is not. I would run it again.
 
Huh?

You said plugs were removed, then how was compression check done? With 5/80 sounds like the lower plug was not installed on the cylinder undergoing check. Or the cylinder was not at TDC. JMHO.
 
Mike Busch claims that compression readings have no correlation at all to an engines ability to make full rates power. It only tells you if the engine will consume oil.

Continental says to borescope the cylinders to determine their condition. If that is okay then fly for (??) hours and then redo the compression test.

Don
 
You're on the right track listening for air leaks. Air flow sounds from exhaust pipe or fuel control will id valve issues... leak sounds through the crankcase breather id rings/piston/cyl issues. Given the "mid 70s" reading on cyl 3 check, air flowing in #1 and out #3 is (understatement to follow) difficult to explain. Good Luck.
 
You said plugs were removed, then how was compression check done? With 5/80 sounds like the lower plug was not installed on the cylinder undergoing check. Or the cylinder was not at TDC. JMHO.

No - one plug from each cylinder was removed to perform the compression check.

Four different eyes and two compression test rigs were used to verified the 5/80 reading was correct.

Carl
 
You're on the right track listening for air leaks. Air flow sounds from exhaust pipe or fuel control will id valve issues... leak sounds through the crankcase breather id rings/piston/cyl issues. Given the "mid 70s" reading on cyl 3 check, air flowing in #1 and out #3 is (understatement to follow) difficult to explain. Good Luck.

I agree - we are befuddled. On the air coming out #3, the #1 cylinder had the compression check rig connected and was at TDC with both valves shut. I believe the #3 cylinder had the intake valve open at this point (but I'd have to verify).

Carl
 
Low compression

Carl

I had a similar experience with my RV7 (IO-360) this year. Last November I tested the compression and got #1 79/80, #2 80/80, #3 77/80, #4 75/80. In February 2017, airplane was in paint shop for about a 6 weeks. After flying the airplane home in March, I did my condition inspection in and got #1 75/80, #2 38/80 (could hear leak in oil tube), #3 30/80 (could hear leak in oil tube) #4 0/80 (could hear leak in exhaust). Prior to this the engine started fine, ran smooth with normal CHT, EGT, fuel flow, RPM, MP as far as I could see.

After the low compression test, I did a ground run for about 10 minutes and had a friend recheck compressions (engine warm) with his gauge and got #1 75/80, #2 35/80, #3 30/80, #4 60/80. (Another friend repeated the compression test about a week later with about the same result.)

We also checked the three low cylinders with the borescope, confirmed the valves were opening and closing; exhaust valves looked OK to us, and no major anomalies were found on any of the valves, cylinder walls, or anywhere else in these 3 cylinders.

At this time the engine had ~180 hrs., and uses 1 qt of oil per ~12 hrs. I generally cruise at 65% hp, LOP, where the CHT range (highest to lowest) is about 30 deg, and the EGT range is about 25 deg.

I consulted with a local engine shop (who rebuilt my C145 in my C-170 about 15 years ago), and he recommended flying the airplane about 20 ? 25 hrs and rechecking. I did that and the compressions came up to #1 76/80, #2 65/80, #3 64/80, #4 73/80. The engine runs fine, so I don't plan on checking it again until the next condition inspection.

Interestingly, I attended a forum How to Maximize Engine Longevity at Osh 2017 by guy from Ram Aircraft who emphasized the importance of regular aircraft operation to combat internal engine corrosion. Moisture that condenses on internal surfaces can sit there resulting in significant corrosion unless the aircraft is flown regularly, or other steps taken to remove moisture from the cylinders and crankcase.

As I do not fly my airplane but a few times per month, I have a DIY desiccator system that I now connect to the engine crankcase and both exhaust pipes while it sits in the hangar. Time will tell if this has any benefit what so ever.

Warren
RV-7
KMIC
 
I'm helping a neighbor work and engine problem. The plane has been idle for several months and he is getting it ready for conditional inspection. The compression check for #1 cylinder is 5/80, the other three cylinders are in the mid 70s. There is perhaps 300 hours on this engine.

Bore scope showed nothing of interest. The valves and valve seats look fine, and both valves are fully closing. The cylinder walls looked normal. I could find no cracks on the cylinder head or barrel (some areas are obscured).

On engine run up the engine is smooth and all four cylinders CHT and EGTs are tracking (as in normal temps). After a long high (for the ground) RPM run we did the compression check again and it was still at 5/80.

I can hear the air leaking out of the cylinder and noted that a little air was coming out the #3 cylinder spark plug hole (plugs were removed for compression checks).

My only guess are the rings have failed - but I would have expected to see some evidence of this on the cylinder walls.

Other ideas?
Recommendations for next step?

Carl

Carl, I think you mean a leak down check, right? Pressure at TDC with a fixed supply orifice?

If so, then where did you hear the "leak" Intake manifold, exhaust or breather?
The source of the sound points clearly to the leak cause.

Was the engine run and warmed before testing? If so, and it is running smoothly, and if the leaking was a valve, then run some more and recheck. I am assuming it was a valve, as I can not imagine how crankcase pressure would push air around the piston up into another cylinder. I could imagine an intake valve leak doing this.

It is also possible that, if it was cold, a ring has stuck and will release when the piston gets hot - a ground ground run is not sufficient. And, no, it is not always a direct relation between oil consumption and "rings". Bad compression rings but good oil control rings will have more blow-by and low ratio on leak down test, but can still have decent oil control.

In short, engines have many variables that lead to measurable changes.

I am going to guess (please confirm) that a valve was not fully seating, and if it is the intake, then with your borescope inspection of cylinder walls, some flight time would be appropriate to load the engine and then retest. Full power climb and high %power cruise for a 30 min should be a good start.
 
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I'm helping a neighbor work and engine problem. The plane has been idle for several months and he is getting it ready for conditional inspection. The compression check for #1 cylinder is 5/80, the other three cylinders are in the mid 70s. There is perhaps 300 hours on this engine.

Bore scope showed nothing of interest. The valves and valve seats look fine, and both valves are fully closing. The cylinder walls looked normal. I could find no cracks on the cylinder head or barrel (some areas are obscured).

On engine run up the engine is smooth and all four cylinders CHT and EGTs are tracking (as in normal temps). After a long high (for the ground) RPM run we did the compression check again and it was still at 5/80.

I can hear the air leaking out of the cylinder and noted that a little air was coming out the #3 cylinder spark plug hole (plugs were removed for compression checks).

My only guess are the rings have failed - but I would have expected to see some evidence of this on the cylinder walls.

Other ideas?
Recommendations for next step?

Carl

A stuck rings will show no visible signs on the cylinder wall. a cracked ring may or may not show visible damage on the cylinder wall. Given that you felt air near the plug and hear air outside the cylinder, I would also be looking for a cracked cylinder head or separation from the steel barrel. Don't know the weak spots for the Lyc heads. You also need to listen to the breather outlet, the exhaust pipe the the carb throat to see where the air is departing. That will tell you where to look further.

EDIT: Sorry, I mis-read your post. If you were testing #1 and air came out the sparkplug hole on #3, that would indicate the leak is the intake valve (assuming you had the throttle fully closed - air can't easily get out of the manifold and goes to the cylinder with it's intake valve open and then through the sparkplug hole. I could be just a carbon chunk keeping it from closing. An old trick is to pull the rocker and give the valve tip a few whacks with a soft (i.e. brass) hammer.

Larry
 
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You could also verify that the plug for the primer inlet is still there. I have seen that missing before.
Also, check for cracks between the spark plug holes and verify the injector is tight.
You didn't say what engine, but some Titan cylinders require regular inspection to verify that the head isn't loosening. Just sayin'
 
It sounds to me like you have a leak in either an exhaust valve or intake valve on #1 and the #3 cylinder exhaust or intake valve is open so you are hearing the air leaking out of the #1 valve, through the intake or exhaust manifold and out the open valve in the #3 cylinder.
 
In the first post you said you can hear air @ the spark plug, remove that plug feel the seat with your finger dose it feel good and smooth or snaggey/ruff .A louse pug can erode the seat also if plug seized to the helicore insert it could have backed out a bit. Temporarily for compression check only replace copper washer with an O-ring with a good slavering of fuel lube for a good seal and do the differential pessure test again.
 
You might want to bleed the hydraulic assemblies and check the valve lash on those cylinders. It's possible they were not set correctly and with normal wear the lash is gone and the valves are not seating fully.
If I'm not mistaken, this is suppose to be done after the first 200-300 hrs anyway.
Tim Andres
 
Thanks

I appreciate the thoughtful responses.

I too assumed the intake valve on #1 was the culprit (not fully seating). Reading reports of this happening on Lycoming engines however all had the same pre-conditions - the engine ran very rough. This engine runs smooth as glass and considering the EGTs and CHTs are tracking with the other cylinders, we conclude #1 is making power.

I do know that Continental did a test on what happens when piston rings are filed short such that the compression check is 0/80. The engine still produced full power.

The airplane owner (a Lancair 360) has sent all the data to the engine shop that built this engine for their recommendation. I suspect we'll be pulling that jug just to make sure.

Carl
 
I know of 2 lanceairs from Dogwood. One's a world record setter, the other is an AME for NASA is it one of these gentlemen?
 
I appreciate the thoughtful responses.

I too assumed the intake valve on #1 was the culprit (not fully seating). Reading reports of this happening on Lycoming engines however all had the same pre-conditions - the engine ran very rough. This engine runs smooth as glass and considering the EGTs and CHTs are tracking with the other cylinders, we conclude #1 is making power.

I do know that Continental did a test on what happens when piston rings are filed short such that the compression check is 0/80. The engine still produced full power.

The airplane owner (a Lancair 360) has sent all the data to the engine shop that built this engine for their recommendation. I suspect we'll be pulling that jug just to make sure.

Carl

You are correct. Leak down testing is not a good test for gauging performance issues. In the automotive world the go to test is a dynamic compression test, measuring the actual compression, in PSI, produced by the cylinder while spinning. Leak down tests are a secondary test to help pinpoint specific problems and provide a general guide to cyl wall / ring interface condition. As you have seen, a cylinder with poor leak down results can still produce solid dynamic compression when the source of the leak is ring/wall. This is because of the static environment. Once the piston is moving, the leakage is reduced and also less relevant.

The fact that you were making good power on the cylinder was the reason I suspected stuck rings. They typically still produce enough power to be unnoticeable.

Larry
 
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You are correct. Leak down testing is not a good test for gauging performance issues. In the automotive world the go to test is a dynamic compression test, measuring the actual compression, in PSI, produced by the cylinder while spinning. Leak down tests are a secondary test to help pinpoint specific problems and provide a general guide to cyl wall / ring interface condition. As you have seen, a cylinder with poor leak down results can still produce solid dynamic compression when the source of the leak is ring/wall. This is because of the static environment. Once the piston is moving, the leakage is reduced and also less relevant.

The fact that you were making good power on the cylinder was the reason I suspected stuck rings. They typically still produce enough power to be unnoticeable.

Larry

The compression test (rotating) is a less repeatable and less sensitive test than a leak down test. I always counted the compression strokes to compare cylinders.

If it was a stuck ring (certainly one of a general list of possibilities), the "leak" would not blow air out another open spark plug.

Let's be careful about supporting idea that a bad leak down test is not relevant even if a compression test is good. While one might fly home, a small leak certainly may not affect idle or power - yet, but - it should be heeded as something to investigate. "Investigate" means it can produce false results, but that needs to be verified. In this case, it could be a piece of carbon under the intake valve or a tight valve at the limit of the lash adjuster. A list of things could cause a tight valve.

Good thing to get the engine builder involved at relatively low hours.
 
Mike Busch claims that compression readings have no correlation at all to an engines ability to make full rates power. It only tells you if the engine will consume oil.

Continental says to borescope the cylinders to determine their condition. If that is okay then fly for (??) hours and then redo the compression test.

Don

See https://www.aopa.org/news-and-media/all-news/2017/may/pilot/savvy-maintenance-borescope
A compression test measures static compression. You slowly bring up the pressure and hold it, but that's not what happens when your engine is running. During the compression stroke the piston is moving very rapidly and while I'm sure the underlying physics are more complicated, there essentially isn't time for enough air to leak. 5/80 is pretty low and if you can't bump it higher by pushing on the prop then you might have a more serious leak (like the loose plug that someone mentioned), but if the borescope looks good and the engine runs fine then you are probably just going to go through a lot of oil. There might another problem that someone more knowledgeable can point to, but a compression test is not a reliable or repeatable test and shouldn't be used as the basis for airworthiness.
 
Mike Busch claims that compression readings have no correlation at all to an engines ability to make full rates power. It only tells you if the engine will consume oil.

Continental says to borescope the cylinders to determine their condition. If that is okay then fly for (??) hours and then redo the compression test.

Don

I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.
 
What?

See https://www.aopa.org/news-and-media/all-news/2017/may/pilot/savvy-maintenance-borescope
A compression test measures static compression. You slowly bring up the pressure and hold it, but that's not what happens when your engine is running. During the compression stroke the piston is moving very rapidly and while I'm sure the underlying physics are more complicated, there essentially isn't time for enough air to leak. 5/80 is pretty low and if you can't bump it higher by pushing on the prop then you might have a more serious leak (like the loose plug that someone mentioned), but if the borescope looks good and the engine runs fine then you are probably just going to go through a lot of oil. There might another problem that someone more knowledgeable can point to, but a compression test is not a reliable or repeatable test and shouldn't be used as the basis for airworthiness.
Are you saying to Push on the prop during a compression test? I hope not this could be dangerous. But I had a bad test because the rings were not seated as if they were on a normal compression stroke, but never have seen 5/80. I am really curious what caused this. Eating my popcorn while I wait.
 
Are you saying to Push on the prop during a compression test? I hope not this could be dangerous. But I had a bad test because the rings were not seated as if they were on a normal compression stroke, but never have seen 5/80. I am really curious what caused this. Eating my popcorn while I wait.

Is it dangerous? Are you just worried about the general dangers of non-grounded magneto accidental start or is there something else? By "push" I just mean move the prop slightly to add some pressure.
 
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With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.

But.

As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.

I agree with doing a very slight bump. It takes very little to make the gauge jump a bit. Just to make sure the gauge is repeating and not sticking. Anything more than a "tiny slight bump" is asking for painful and scary injuries, or worse.

I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.
 
With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.

But.

As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.

I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.

I think you forgot that the prop is a long lever arm. 1600 lbs at the crank journal, which is offset, what, maybe 4", from the centerline?, will turn into 160 lbs at the end of a 40" radius prop. Still nothing to sneeze at. But not 1600 lbs. In fact, 80 psi is standard for these tests because it was thought that that was the maximum "safe" pressure one should use.
 
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.

IIRC Mike B was talking specifically about leakage past the rings. He has long advocated that NO leakage should be tolerated past a valve (this was at a time when TCM was having exhaust valve issues, and came up with the idea that some leakage was not grounds for a warrantee claim).
 
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.

You would see a burnt valve on the borescope so I'm not sure that there is a qualification. He's not saying that you should throw out the compression check and just hope for the best, he's saying that a borescope is a much better tool.
 
With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.

But.

As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.

I agree with doing a very slight bump. It takes very little to make the gauge jump a bit. Just to make sure the gauge is repeating and not sticking. Anything more than a "tiny slight bump" is asking for painful and scary injuries, or worse.

I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.

Since it's been pretty well established that the raw compression values don't have a lot of meaning, is there a compelling reason to continue to use 80 PSI? Maybe 40 or even 20 PSI would work just as well?
 
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.

I agree, these articles are not gospels but some technical content written for interesting effect. The do contain some interesting facts. The Conti test increased gaps - we both know the ring pressures are highest on the top and decrease for second compression and the oil ring pressure is little affected by the chamber pressures. Meaning, the wear and gap increases (and bore wear) are not the same for all rings. If the Conti test evenly increased all the gaps, then the oil consumption could easily have increased due to uncovered area by the gap in the oil rings.

If one wanted to follow a diagnostic flow chart it would likely fill a double garage door with 10 point type and enough lines to make the reader blind. There are few simple truths in engine diagnostics. PRB technology is just a part of the whole engine.
 
Lycoming SI 1191A

Instead of all the conjecture about how and what your compression test indicates, possibly consult Lycoming's service instruction SI 1191A. While not regulatory, Lycoming probably has a better understanding of their engine design than most other conjectures.

Do your research - Lycoming and TCM cylinder compression tests have significantly different processes.

Again, not regulatory, but Lycoming and AC 43.13B have similar compression test processes which recommend that if a static compression is below 60 psi, further investigation should be considered. 5 / 80 psi is a dead give away for a massive leak somewhere. Possibly a valve is burned or crack in the cylinder. Leaky rings show up as air hissing out the oil dip stick or breather, air out the exhaust or intake. Spray the cylinder head / barrel with soapy water solution to look for cracks in the head / barrel.
 
Instead of all the conjecture about how and what your compression test indicates, possibly consult Lycoming's service instruction SI 1191A. While not regulatory, Lycoming probably has a better understanding of their engine design than most other conjectures.

Do your research - Lycoming and TCM cylinder compression tests have significantly different processes.

Again, not regulatory, but Lycoming and AC 43.13B have similar compression test processes which recommend that if a static compression is below 60 psi, further investigation should be considered. 5 / 80 psi is a dead give away for a massive leak somewhere. Possibly a valve is burned or crack in the cylinder. Leaky rings show up as air hissing out the oil dip stick or breather, air out the exhaust or intake. Spray the cylinder head / barrel with soapy water solution to look for cracks in the head / barrel.

We have a lot more data now and I think what it has shown is that some of these recommendations were not evidence-based. Is 5/80 a problem that shouldn't be ignored? Absolutely, but I want engine maintenance to be science, not dogma. Consider TBO for example. If you want to increase your odds of a catastrophic failure then by all means overhaul an engine that is running well and shows no signs of major problems. On its face it seems like a conservative recommendation and in a perfect world it might be, but we have plenty of data now to show that the first 200 hours on a new or overhauled engine have the highest risk of catastrophic failure. Every time you take something apart you also increase the risk that the person doing the assembly/disassembly will make a mistake. Lycoming is unlikely to change their recommended practices because those changes require FAA approval.
 
I don't know what is causing this problem, but I do know that if there is a leak at an exhaust flange it will erode the interface. I would expect if there is a leak at valve it would also erode the valve seat. If there is a stuck ring, because the engine has not been run regularly then it might free up, or it might not. My point is that there is "potential" for something that will damage the engine, but it might not be anything serious. I would certainly keep investigating.

I think Mike Busch is saying that the compression test can show a bad result which can mean either something trivial or something major - you just don't know, so don't go ripping off a jug and undoing through-bolts just based on that. He certainly isn't saying don't worry, be happy. Fly for a few more hours, borescope, check the filter for any metal, redo the compression test, do a very careful visual inspection around every possible leak location looking for soot etc. Listen for where the air is leaking out - it's gotta go somewhere. Eventually something will turn up or it will all go back to normal. But you probably don't want to head out over to Russia like Vlad until you have satisfied yourself that you know what it is. Please let us know.
 
With 5/80 you have a horrendous leak that ought to be obvious and easily located in the intake, exhaust or crankcase vent. With those numbers there will be a torrent of wind coming out somewhere. Pretty simple to find with those kind of numbers. How about the possibility you are on the valve overlap position instead of TDC?
Tim Andres
 
With 5/80 you have a horrendous leak that ought to be obvious and easily located in the intake, exhaust or crankcase vent. With those numbers there will be a torrent of wind coming out somewhere.
Tim Andres

The air goes through an .040" orifice. There is never that much air flowing. Dial up your unit to 80 psi and hold your thumb over the end to produce 5/80. You can't even feel the air moving once 5" from the exit. It is far from a "torrent of wind."
 
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The air goes through an .040" orifice. There is never that much air flowing. Dial up your unit to 80 psi and hold your thumb over the end to produce 5/80. You can't even feel the air moving once 5" from the exit. It is far from a "torrent of wind."

I think you have that backwards. 75 psi is leaking, not 5 psi.
 
I think you have that backwards. 75 psi is leaking, not 5 psi.

5/80 means 5 PSI on the post-orrifice gauge and 80 PSI on the source gauge. Yes, the 5 PSI reading is 75 PSI less than the measurement with no leakage at all. That is what I was referring to in my post. Eventhough it is very large movement of measurement on the instrument, it doesn't translate to very large movement of air.

I suggested setting the source gauge to 80 PSI with an open end. The second gauge will show 0. Apply a bit of resistance to the air outlet and the second gauge will start going up. Stop when it reaches 5 (the OP's reading) and feel how much air is flowing. You will find it is not significant and definitely not a torrent.
 
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